the hydrogeological aspects of shale gas extraction in the uk

The Hydrogeological Aspects of Shale GasExtraction in the UK

ROBERT S. WARD,* MARIANNE E. STUART AND JOHN P. BLOOMFIELD

ABSTRACT

The UK may possess considerable reserves of shale gas underlying asignificant proportion of the UK, but as yet there has been very littleexploratory drilling to confirm the resource potential. The areas likelyto be exploited for shale gas are overlain in many areas by aquifers usedfor drinking water supply and for supporting baseflow to rivers. Thevulnerability of groundwater and the wider water environment musttherefore be taken very seriously. Experience from the United Statessuggests that groundwater may potentially be contaminated by ex-traction of shale gas, both from the constituents of shale gas itself,from the hydraulic fracturing fluids, from flowback/produced waterwhich may have a high content of saline formation water or fromdrilling operations. A rigorous assessment of the risks is required andappropriate risk-management strategies developed and implemented ifthe industry is to become established in the UK. It is likely that, due toenvironmental sensitivities, there will be some locations where shalegas exploitation will be considered unacceptable and this may affectthe economic viability of the industry. Because we are still at a veryearly stage, we can take advantage of experience where things havegone wrong elsewhere and ensure progress is made in a controlled way.We must identify and understand the risks to groundwater from shalegas and establish a fully informed risk management strategy for the

121

Issues in Environmental Science and Technology, 39FrackingEdited by R.E. Hester and R.M. Harrisonr The Royal Society of Chemistry 2015Published by the Royal Society of Chemistry, www.rsc.org

*Corresponding author.

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industry. We must not look back in 2030 years and regret not takingthe actions we have the opportunity to take now.

1 Introduction

With the increasing demand for gas in the UK, declining North Sea gas re-serves and the drive for greater energy security, attention is turning to al-ternative domestic sources of gas. One of these is shale gas. Resource studiesfor the UK are not yet complete, but early indications are that the UK maypossess considerable reserves.1 However, very little exploration has takenplace and so its potential as an economically viable source of gas has yet tobe determined. Over the next decade it is expected that exploration activitywill significantly increase as a pre-cursor to exploitation and an even greaterlevel of industrial activity.There are significant technical challenges ahead for an industry that has

no track record in the UK and these will apply during both the explorationand exploitation phases. The process of shale gas extraction (which, for thepurposes of this chapter, includes both exploration and commercial ex-ploitation) involves accessing gas-rich shale at considerable depth below theground surface and then hydraulically fracturing (stimulating or fracking)the rock. This controlled fracturing significantly increases the permeabilityof the shale by creating fissures and interconnected cracks in material thatnaturally has extremely low permeability. As a result, gas trapped in the rockis released and can flow into the well and then to the surface.Hydrogeological considerations play a very important role in shale gas

extraction for a number of reasons. Significant volumes of water are requiredto drill and hydraulically fracture the shale and some of this will need to besourced from groundwater. The drilling of wells from the surface to depthsof typically a kilometre or more requires penetration through geologicalformations near to the surface that contain freshwater (groundwater). Thisgroundwater may be used as a source of water for drinking, for industry(including food production and agriculture) and for supporting stream flow,groundwater-fed wetlands and their associated ecosystems. The drilling ac-tivity itself may impact on the quality of groundwater if not managed ef-fectively, but the greatest concern arises from the potential contamination ofgroundwater (and the wider environmental impact) by the constituents inthe fluid used to hydraulically fracture the shale, the water that returns to thesurface after the fracturing operation (flowback or produced water) andthe constituents of the shale gas.Whilst there are already well-developed regulatory regimes in the UK for

hydrocarbon operations and for groundwater protection, it is not yet clearhow effective they will be for a new and unproven shale gas industry in theUK. This chapter examines some of the key considerations for shale gasextraction in the UK in relation to groundwater management and protection,taking into account experience elsewhere in the world where the industry isalready established.

122 Robert S. Ward, Marianne E. Stuart and John P. Bloomfield

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2 Potential Shale Gas Resources and Aquifers in the UK

2.1 Potential Shale Gas Source Rocks

The establishment of a successful shale gas industry in the United States hasled to many other countries considering the potential within their own ter-ritories. The UK is one of these and, with support from the current Gov-ernment and the introduction of economic incentives, a significant amountof exploration is expected over the coming decade. This exploration is nee-ded ahead of any commercial development, as very little is currently knownabout how much shale gas can be extracted and whether it can be producedeconomically.It was over 20 years ago that it was suggested that the UK may have

abundant shale gas reserves, but at that time there was little interest asNorth Sea gas was still plentiful and the technology to extract the gas was inits infancy. There is now significant interest, with growing recognition thatthere may be considerable on-shore shale gas potential. Originally it wasassumed that this potential was restricted to locations where there hadbeen thermal maturation of organic-rich shales to produce (thermogenic)gas. However, it is now known that (biogenic) shale gas can also be formedby microbiological degradation of organic material irrespective of geo-logical age and burial depth.2 This finding enhances the UK shale gasresource potential dramatically, making many more rocks potentiallyprospective.Potential shale gas source rocks occur in many areas of the UK,

including the Carboniferous shales in the Midland Valley of Scotlandand across Northern England (Pennine Basin, Stainmore and Northumber-land Basin system and Widmerpool Trough), the Jurassic shales in theWessex and Weald Basins and Lower Palaeozoic shales associates with theMidland Microcraton that extends from Wales in the west to the ThamesEstuary in the east.3 Whilst the UK has an abundance of shale, there isconsiderable variation in the depth and thickness of each of the shaleformations and their full extent is not yet known. The potential shale gassource rocks that are currently being considered in the UK are shown inTable 1.The first of a series of detailed shale gas resource estimates for UK shales

was published in 2013.1 This focussed on the Carboniferous Bowland-Hodder unit (Bowland Shale) across Northern England. A total gas-in-placeresource estimate was made using a 3D-geological model based on over15 000 miles of seismic profile data integrated with outcrop mapping andinformation from 64 deep boreholes. The study showed that the BowlandShale can be divided into an upper and lower unit. The lower unit is struc-turally more complex than the upper unit, which is considered to be moresimilar to the Barnett Shale in the United States. The relatively complexlower unit is unlike anything encountered in the United States and it is in-ferred that there will be unique challenges for the shale gas industry in theUK if it is to be successfully developed.

123The Hydrogeological Aspects of Shale Gas Extraction in the UK

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The study applied a statistical approach to assess the resource, which tookinto account variations in the input parameters. As a result, the gas resourceestimates are provided as a range with upper (P10), lower (P90) and median(P50) values (see Table 2).With the UK shale gas in its infancy and with very little exploration activity

to date, it is currently too early to make any reliable estimate of the Tech-nically Recoverable Resource (TRR) or Reserve figure (the proportion of theTRR that is commercially recoverable).

Table 1 UK shales of interest to shale gas extraction (summarised from Andrews1

and from Harvey and Gray).4

Province Basin Source shale Thickness (m) Comment

Scotland Midland ValleyBasin

Strathclyde Group,Carboniferous

Up to 670 Immaturefor oil

CentralBritain

Bowland Basin BowlandHodderUnit, CravenGroup,Carboniferous

Typically 150but reaches890

First shale ofinterest basinswithin gaswindow

Edale BasinWidmerpoolTrough

GainsboroughTrough

Cleveland Basin

WessexWealdProvince

Wessex Basin Lias, LowerJurassic

Thin beds ofoil shale

Wytch Farmsource rock

Weald Basin Kimmeridge Clay,Upper Jurassic

Over 600 incentre

Immature.Probablybiogenic

Midlands Microcraton Tremadoc Shales,Upper Cambrian

Uncertain

N E EnglandProvince

Northumber-land Trough

Yoredale,Carboniferous

Shale unitstend to bethin

StainmoreTrough

South Wales-Bristol Basin Marros Group,Carboniferous

Uncertain Interbeddedwith thicksandstones

Table 2 Shale gas resource (gas-in-place) estimate for the Bowland Shales.1

Total shale gas resourceestimate (tcf)

Total shale gas resourceestimate (tcm)

Low(P90)

Central(P50)

High(P10)

Low(P90)

Central(P50)

High(P10)

Upper unit 164 264 447 4.6 7.5 12.7Lower unit 658 1065 1834 18.6 30.2 51.9Total 822 1329 2281 23.3 37.6 64.6

(Units tcf and tcm are trillions of cubic feet and trillions of cubic metres, respectively).


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2.2 UK Aquifers

The UK obtains around 30% of its public water supply from groundwater,with most of this water abstracted from the principal highly productivebedrock aquifers. Examples include the Chalk of Southern and EasternEngland and the Permo-Triassic sandstones in the Midlands. However, it isnot only the public supply aquifers that are important. Many tens of thou-sands of private (drinking water and industrial) supplies abstract from sec-ondary (or moderately productive) aquifers and groundwater plays a vitalrole in maintaining the baseflow in rivers and supplying water to wetlandsand the ecosystems that are dependent on this water. Figure 1 shows thedistribution of highly and moderately productive aquifers across the UK.Whilst these may be the most productive aquifers, groundwater in otherareas (poorly productive aquifers) may also be locally important for baseflow,wetlands and private water supply.Two of the biggest concerns related to shale gas extraction are ground-

water contamination and over-abstraction of water. As a new industry in theUK, these issues must be taken very seriously. Fortunately, the UK, withhighly developed and mature groundwater legislation, management/protection policies and supporting tools, is in a strong position. This is incontrast to the United States where regulation at the start of their develop-ment of shale gas was limited and where groundwater issues are now arising(see later). The UK Government(s) and their environment agencies regulateeffectively all potentially polluting industries and, in this context, a nascentshale gas industry will be subject to exactly the same environmental regu-lation. However, as it is not yet clear how the industry will develop, it is stilluncertain what any specific challenges will be in the UK environmentalsetting. We know from the past that poorly regulated and uncontrolled in-dustrial activity can lead to long-term environmental problems and costlyremediation. Examples include the contaminated land associated with for-mer gasworks, fuel stations, mining and waste disposal before effectiveregulation was brought into force.Ahead of any new activity that may be potentially polluting, there is a need

to fully consider the risks associated with it, both in terms of health andsafety to humans and to the environment. Jackson et al.5 identify two areaswhere research is needed ahead of any development of unconventional gasextraction: baseline monitoring and characterisation of pathways andmechanisms by which contaminants may potentially pollute surface/nearsurface water resources. In the UK, the British Geological Survey (BGS) hasalso recognised this as being important and is carrying out baseline moni-toring in those areas that have been identified for shale gas exploration6 andseparately is developing (jointly with the Environment Agency) a 3D-model ofthe spatial relationship between potential shale gas source rocks and theprincipal aquifers in England and Wales. This work uses the BGS 3D Geo-logical Model of Great Britain7 and the Aquifer Designation dataset.8 Thefull extent of each rock type (shale and aquifer) has been mapped and/or


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modelled and the vertical separation calculated for each shale gas sourcerock and aquifer pair. The relationship between the different aquifers andshales is shown in Table 3. Based on this modelling, approximately 30% of

Figure 1 Aquifers of the UK that have high and moderate productivity.


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Table 3 Relationship of aquifers to potential shale gas targets.9

Aquifer Shale gas source Area where both present

Thickness ofintervening strata(m)

Crag Kellaways/Oxford/Osgodby Clays East Anglia 370460Crag Upper Lias Norfolk 350530Chalk Kimmeridge/Ampthill Clays North & South Downs, Hampshire,

Wiltshire, Dorset501200

Chalk Kellaways/Oxford/Osgodby Clays Lincolnshire, Norfolk, North & SouthDowns, Hampshire, Wiltshire, Dorset

111500

Chalk Upper Lias Yorkshire, Lincolnshire, Norfolk, North &South Downs, Hampshire, Wiltshire,Dorset

301900

Lower Greensand Kimmeridge/Ampthill Clays North & South Downs, Hampshire,Wiltshire, Dorset

2001400

Lower Greensand Kellaways/Oxford/Osgodby Clays Lincolnshire, Norfolk, Cambridge,Bedford, Bucks, Berks North & SouthDowns, Hampshire, Wiltshire, Dorset

371800

Lower Cretaceous sandstones-Spilsby Kellaways/Oxford/Osgodby Clays Lincolnshire 250500Lower Cretaceous sandstones-Spilsby Upper Lias Lincolnshire 300600Corallian Kellaways/Oxford/Osgodby Clays Wessex-Weald, Wiltshire 3001100Corallian Upper Lias Yorkshire, Wessex-Weald, Wiltshire 1001600Jurassic Oolitic limestones Upper Lias Lincolnshire, Norfolk, Cambridge,

Bedford, Bucks, Berks North & SouthDowns, Hampshire, Wiltshire, Dorset

1901700

Triassic sandstones Bowland-Hodder West Cumbria, Lancashire, Cheshire,Derbyshire, East Midlands, Lincolnshire,Yorkshire

05000 Thinnestin South Mid-lands and thick-est in CheshireBasin

Permo-Triassic sandstones Upper Cambrian shales Warwickshire 01300Permian sandstones Bowland-Hodder West Lancashire and Cheshire Basin 05000Permian (Magnesian) limestone Bowland-Hodder Yorkshire, Lincolnshire 801800None Marros NA NANone Cambrian Shales NA NA

127The

Hydrogeological

Aspectsof

ShaleGas

Extractionin

theUK

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England and Wales is underlain by potential shale gas source rocks and 50%by geological formations designated as Principal Aquifers (see Figure 2).The principal groundwater issues thought to be important when con-

sidering shale gas exploration and extraction are considered in the followingsections. These have arisen from the experience that has emerged fromelsewhere in the world, especially the United States where shale gas ex-ploitation is now well established.

3 Water Resources

The drilling and completion of shale gas wells can require large quantities ofwater, as drilling the vertical and horizontal components of the well requirewater for maintaining hydrostatic pressure, lubrication and cooling of thedrill bit, and to return the cuttings to the surface. A further larger volume ofwater is then needed to carry out the hydraulic fracturing process.A review of literature associated with shale gas well drilling and stimu-

lation indicates a wide range in the values reported for water use. Thisvariation generally reflects the complexity of the drilling, the geologicalconditions encountered, total depth of the well and length of the horizontalsections and the number of hydraulic fracturing stages. Estimates of thewater requirements for drilling and hydraulic fracturing in different shalegas areas (plays) in the United States, are shown in Table 4.10 For com-parison, the figures quoted by Cuadrilla for the drilling and hydraulic frac-turing of their Preese Hall exploratory well in Lancashire are also shown.11

These figures are lower than those from the United States because they only

Figure 2 Distribution of (a) shale and (b) aquifers across England and Wales.


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reflect the water used for a single exploratory borehole rather than multi-stage hydraulically fractured production well.It is therefore very difficult to estimate how much water will be required in

the UK, and in different locations, for shale gas operations because of thesignificant uncertainty about how the industry may develop and how much itwill differ from that elsewhere in the world. The only practical way to con-sider the potential implications is to examine a number of possible scen-arios. This has been done as part of the Strategic Environmental Assessment(SEA) for unconventional gas development in the UK, where a range of be-tween 10 000 and 25 000 m3 has been considered.12 The SEA considered twoactivity scenarios: high and low development, each with a number of as-sumptions being made that resulted in estimates of the total number ofproduction wells ranging from 180 to 2880 and each requiring to be re-fractured once during its lifetime. The resulting range for total water re-quirement was between 3.6 and 144 million m3.This water would not all be needed at the same time or in the same lo-

cation and so an additional major complication is the rate at which the wellswould be drilled and hydraulically fractured, and where they will be. Sincethis information is unavailable as yet, any estimates of water demand forshale gas in the UK are purely speculative. However, if we consider one de-velopment scenario proceeding with 100 wells being drilled and completedeach year this would require 2.5 million m3, based on the maximum waterusage used in the SEA. This is a large number, but how does it compare tohow much we already use each year? The most recent UK Governmentstatistics on water abstraction (www.gov.uk) for 2012 estimated that the totalnon-tidal freshwater (surface water and groundwater) abstraction forEngland and Wales was 11 700 million m3. This would suggest an overalldemand for shale gas equivalent to approximately 0.02% of overall annualabstraction. A breakdown of the most recent water use estimates for Englandand Wales is shown in Table 5.Overall shale gas requirements are relatively modest, but the challenge

comes from sourcing the water and transporting it to the site at the timerequired. Whilst some areas of the UK have plenty of water others do not,and there may already be significant stress on water resources in these areasand little, if any, room for additional demand. This is particularly (although

Table 4 Estimated water requirements per well for drilling and fracturing in dif-ferent shale gas plays (from Mantell10 and Cuadrilla Resources).11

Shale Play Drilling (m3) Fracking (m3) Total (m3)

Barnett (US) 950 14000 14950Haynesville (US) 2300 19000 22300Fayetteville (US) 250 19000 19250Marcellus (US) 400 21000 21400Eagle Ford (US) 500 23000 23500Bowland Shale (UK)11 900 8400 9300


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not exclusively) the case in southern and eastern England. A stark reminderof this was during the drought in 2012 when water use restrictions wereintroduced.The environment agencies manage water resources and regularly assess

water availability. In England, where demand and pressure on water re-sources is greatest, a water resource management framework is in placethat aims to balance human demand for water with the needs of the en-vironment.13 Through their catchment abstraction management strategy(CAMS) process the agencies regulate and control both surface water andgroundwater abstraction in an integrated way. As with any other industry,the shale gas industry will be subject to these management controls. Themost recent assessment of water resources by the Environment Agency hasbeen carried out at a more-local scale than previously. An example of one ofthe outcomes is shown in Figure 3. This shows for each water body thepercentage of time that additional water is available for abstraction. Inaddition to this, the Environment Agency considers when and how muchwater may be available for abstraction by considering the relationshipbetween abstraction, river flows and environmental flow needs. To ensureadequate protection of water resources and the environment, resourceavailability is calculated for different flow conditions between high (Q30)and low (Q95) flows.The Environment Agencys assessment indicates that there may be sig-

nificant challenges in sourcing adequate and sustainable quantities of waterin some parts of the country where shale gas exploitation is being con-sidered. This will particularly be the case in the South and East of Englandbut, because of local environmental considerations, difficulties cannot beruled out elsewhere, as Figure 3 shows. The problem is not so great for ex-ploration drilling and testing, but will be significant if industrial-scale de-velopment takes place where large numbers of wells will need to be drilledand hydraulically fractured.

Table 5 Estimated water usage by different users in 2012 (from www.gov.uk).

UseVolume(million m3) Use

Volume(million m3)

Public water supply 4144 Spray irrigation 30Electricity supplyindustry

5702 Agriculture (excl. irrigation) 2

Fish farming, cressgrowing, amenityponds

864 Private water supply 1

Other industry 950 Other uses 10

TOTAL (all uses) 11701

Estimate maximumannual requirementfor shale gas(100 wells per annum)

2.5


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4 Contaminant/Pollutant Sources

Shale gas operations, as with many other industrial activities, involve the useand/or production of chemicals and materials that are hazardous. The risksassociated with these hazards needs to be assessed and managed effectivelyif the activity is to be carried out safely. In managing the risks there is a needto understand the hazards. The focus here is specifically on risks togroundwater, but many of them will also be relevant to different parts of thewater environment.

4.1 Drilling

Shale gas exploration and exploitation requires the drilling of wells from thesurface to the required depth(s). The process of drilling requires the dis-turbance of the ground and in many locations the wells will penetrate

Figure 3 Environment Agency water resource assessment for England and Wales.13


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freshwater aquifers which supply water for human use and/or baseflow torivers. Any structure that penetrates freshwater aquifers, such as a well, hasthe potential to introduce a preferential pathway that could lead to con-tamination of these water sources if pollutants are allowed to leak ormigrate.14

Drilling requires the use of water and drilling mud to ensure that the drillbit is lubricated and cooled and so that the drill cuttings can be returned tothe surface. During drilling, the drill string and bit will be in contact with thegeological formation and groundwater. There is, therefore, a risk ofgroundwater contamination from the drilling mud and/or mobilisation andtransfer of contaminants.The risks associated with well drilling have to be carefully considered

during the planning stage and take into account the purposes of the well, thelocal surface environment and land use (current and past), the potentialreceptors and pollutant/environmental exposure pathways and well design.Control measures must be put in place to mitigate any identified risks. Theseinclude installation of multiple casings to ensure that different geologicalhorizons are isolated and to act as a barrier to leakage of fluid inside thewell; blow-out preventers to avoid over-pressuring in deep boreholes anddamage to the casing; and environmental monitoring.

4.2 Hydraulic Fracturing Fluids

To optimise the recovery of shale gas hydrocarbon source rock, the shales arehydraulically fractured. This involves pumping large volumes of water con-taining around 5% and and 0.5% of chemicals in to the well at high pres-sure. The purpose of the sand (proppant) is to hold open the artificiallycreated fractures and the chemicals to optimise the fracturing process.The exact composition of the fracturing fluid will depend on the oper-

ational conditions, including the geological formation, depth of the well,number of fracturing stages, etc. There is no standard recipe and in de-veloping a mix, different chemicals can be used to provide the same func-tion. The number of chemical additives is also not prescribed and so this willvary as well. A representation of the composition of hydraulic fracturing fluidis shown in Figure 4. This identifies some of the key additives that are oftenused.The viscosity of fresh water tends to be low, which limits its ability to

transport the proppant effectively to achieve a successful fracture stimu-lation treatment. As a result, some hydraulic fracturing fluids have a geladditive (gellant) to increase the viscosity. Gellant selection is based on thehydrocarbon reservoir formation characteristics, such as thickness, porosity,permeability, temperature and pressure. As temperatures increase, thesegels tend to thin dramatically. In order to prevent the loss of viscosity,polymer concentration can be increased (polymer loading) or, instead, cross-linking agents can be added to increase the molecular weight, thus in-creasing the viscosity of the solution.


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The fracturing fluid has to reach the bottom of the well, which may beseveral kilometres below the surface, via a relatively narrow diameter (150200 mm) pipe as efficiently as possible. The addition of friction reducers orsurfactants allows the fluids to be pumped to the target zone at a higher rateand lower pressure. Examples of chemicals used as friction reducers aremethanol or ethylene glycol.Acid is utilised in the beginning of the fracture process to clean-up cement

that is lodged in the well casing perforations and provide an accessible pathto the formation once fracturing fluid is pumped in. Hydrochloric acid ismost commonly used at concentrations ranging from 3 to 28%. In stimu-lations that use acid, a corrosion inhibitor is also often used to hinder thecorrosion of steel tubing, well casing, tools and tanks. The addition of 0.1 to2% of a corrosion inhibitor can decrease corrosion by up to 95%. Concen-trations of corrosion inhibitor depend on down-hole temperatures andcasing and tubing types. At temperatures exceeding 130 1C, higher concen-trations of corrosion inhibitor, a booster or an intensifier may also benecessary. A typical corrosion inhibitor used in shale gas operations isN,N-dimethyl formamide.Biocides are additives that are used to minimise the danger of bacterial

corrosion in the wellbore.15 Fracture fluids typically contain gels that areorganic. They can provide an ideal medium for bacterial growth, reducingviscosity and the ability of the fluid to effectively deliver the proppant. Bio-cides, such as glutaraldehyde are diluted in the fluid in a manner similar tothe addition of the corrosion inhibitor.When a formation contains clay, permeability can be significantly reduced

when exposed to water that is less saline than the formation water. As a

Figure 4 An illustration of the composition of fracture fluid composition for shalegas well hydraulic fracturing operations.


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result, treatment with solutions containing 1 to 3% salt is generally used as abase liquid when clay swelling is probable. Potassium chloride is the mostcommon chemical used as a clay stabiliser due to its ability to stabilise clayagainst the invasion of water to prevent swelling.In some operations, a breaker is also added to the fluid in later stages of

the process to reduce the viscosity of the gelling agent to help release theproppant from the fluid once the fractures have been created and to increasethe volume of flowback water returned to the surface. Chemicals used asbreakers may include magnesium peroxide, peroxydisulfate, sodium per-borate and glycol. Breakers are normally mixed into the fracturing fluidduring the pumping operation to give enough time to transport the proppantto the fracture zone.At the present time there has only been exploratory drilling (and hydraulic

fracturing) for shale gas in the UK, carried out by Cuadrilla at Preese Hall inLancashire. For this they received approval for a limited suite of chemicaladditives which comprised: polyacrylamide (friction reducer), salt (fracturingfluid tracer), hydrochloric acid and glutaraldehyde (biocide).16 At the con-centrations used, these were classed as non-hazardous by the EnvironmentAgency. However, this was only for exploratory purposes and operational wellswill probably need a different suite of chemicals. These would also need to beapproved by the relevant regulator, e.g. the Environment Agency.

4.3 Flow Back and Produced (Formation) Wastewater

A considerable proportion of the fracturing fluid injected into the well re-turns to the surface as flowback. Flowback starts immediately and cancontinue for anything from a few days to a few weeks following hydraulicfracturing. The length of time depends on the geology and geomechanics ofthe formation. The highest rate of flowback occurs on the first day, and therate diminishes over time; the initial rate may be as high as 1000 m3 perday.17 Depending on the geology and extent of fracturing, the volume ofproduced water may range from between 30 and 70% of the injected frac-ture-fluid volume.A certain amount of fluid will continue to emerge from the well over its

entire lifetime. This on-going discharge is termed produced water and itscomposition increasingly reflects that of the geological formation water ra-ther than the injected fluid. The rates of produced-water discharge aregenerally low and the volumes can be relatively easily handled on the sur-face. The principal problem is the potentially large volumes of flowbackwater during the period immediately after hydraulic fracturing.The composition of flowback water will be similar to the injected fluid(s),

modified by the fracturing process and exposure to formation water. It willinclude the chemicals injected, their transformation and/or breakdownproducts and formation water. As an example, concentration ranges for themain components of produced water from Marcellus Shales in the UnitedStates are shown Table 6. A much larger range of trace elements will also be


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present, many of which can be toxic at elevated concentrations. Examplesinclude arsenic, cadmium and nickel. In addition, shales are hydrocarbonsource rocks and so will also contain a range of hydrocarbons and otherorganics such benzene and naphthalene and other oil petroleum-relatedcompounds. They also may contain relatively high concentrations of uran-ium and its associated radioactive decay products. This group of materials isreferred to as NORM Naturally Occurring Radioactive Materials and canlead to a radiological hazard if significant concentrations are contained inthe flowback and/or produced waters. Radium-226 and 228 can be present atup to 1000 pCi g1 (pico curies per gramme) potentially many times over thesafe disposal limit.18,19

The safe handling, storage and disposal/recycling of the wastewater areparamount to avoid risks to humans and the environment. Experience fromthe United States has highlighted the challenges that could be faced in theUK if large-scale shale gas exploitation was to take place. A number of dis-posal routes for wastewater are used in the United States, some of which areunlikely to be allowed in the UK due to much stricter regulations. For ex-ample, direct discharge to surface water would be prohibited.In the United States a large proportion of the wastewater is disposed of

through deep underground injection. Such an option in the UK is likely to belimited by the availability of suitable locations and environmental regu-lation. Other disposal routes will, therefore, need to be considered. Anumber of alternatives have been tried in the US, but none appear to offer asatisfactory solution that could have widespread application in the UK. Forexample, municipal wastewater treatment plants would not be able to han-dle the large volumes of highly saline mineralised water as it would damagethe biological treatment process. Other forms of treatment, such as reverse/forward osmosis and distillation, are possibilities, but both are energy-in-tensive processes and the residual waste, although less in volume, would still

Table 6 Range of constituents in produced water from shale gas wellsextracting from the Marcellus Shales, Pennsylvania, US, afterGregory et al.20

Component Concentration range (mg/l)

Total dissolved solids 66 000261 000Total suspended solids 273200Hardness (as CaCO3) 910055 000Alkalinity (as CaCO3) 2001100Chloride 32 000148 000Sulfate 0500Sodium 18 00044 000Calcium 300031 000Strontium 14006800Barium 23004700Bromide 7201600Oil and grease 10260


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require disposal. Alternative technologies are actively being researched, butto date none appear to offer an adequate long-term solution. An up-to-datereview of treatment technologies is provided by Shaffer et al.21

An option that offers most promise is to re-use the wastewater for sub-sequent drilling/hydraulic fracturing operations. The benefits of re-use areself-evident, but until recently re-use has not been seriously considered. Thisis for a number of reasons, which include the detrimental effect it has onthe behaviour and performance of some of the chemical additives in thehydraulic fracturing fluid, leading to poor operational performance ofthe gas well, and the fact that freshwater is often plentiful and cheap.Increasingly, however, recycling is being considered as regulations are in-creasingly impacting on disposal options, freshwater resources are be-coming limited and technology is advancing.Evidence of this can be seen in shale gas operations exploiting the

Marcellus Shale, Pennsylvania, in the United States. As exploitation hasproceeded, wastewater has been disposed of in a range of ways (see Figure 5).Initially there was an increase in treatment at municipal facilities, but thishas now returned to pre-exploitation levels. The reduction in use of muni-cipal treatment facilities has been driven by a change in regulation, whichmeans that water can no longer be treated in municipal facilities; similarly,limits on discharges from industrial treatment plants have also madetreatment by this method unviable for many plants. In response, there havebeen sharp increases in the use of deep injection and re-use.22

Deep disposal is not permitted in Pennsylvania, so waste water must betransported for considerable distances to use this disposal option and,whilst there has been an increase, there has been a much greater increase inthe volume being recycled as this is now considered to be a more practicaland cost-effective long-term option.

Figure 5 Changes in wastewater management methods for conventional wastewaterand from Marcellus shale exploitation from Lutz et al.22 (Note differencesin vertical scales).


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4.4 Shale Gas

Shale gas is predominantly, but not exclusively, methane. The source of thisgas is the organic material contained in the sediment at the time of de-position. Two processes for the formation of methane are commonly recog-nised: microbial production (biogenic) and non-biological, chemicalproduction (thermocatalytic or thermogenic).23 Biogenic gas is formed atrelatively shallow depths and low temperatures by the anaerobic microbialdecomposition of the organic material. Thermogenic gas is formed at muchgreater depths and over longer geological timescales through the thermalcracking of the organic material at high temperatures and pressures. Ther-mogenic gas is usually associated with the major hydrocarbon reservoirs as oilis formed by the same process. The biogenic process does not produce oil.In general, thermogenic gas has a high methane content with low but

significant concentrations of other hydrocarbons such as ethane (C2) andpropane (C3), with C1/(C2+C3) o100. In contrast, biogenic gas contains aneven higher proportion of methane, with a C1/(C2+C3) ratio between 1000and 10 000. Where a gas contains such a high proportion of methane it isoften described as a dry gas.Biogenic and thermogenic gases can be readily differentiated and char-

acterised through their geochemistry if the gas has remained close to itssource. One method that is often used is the analysis of the stable carbonisotope ratios of the methane (13C/12C). 12C is the most common isotope butaround 1% is 13C. Due to differences in the biological, chemical and/orphysical conditions under which the gas forms, differences in the isotopicratios occur. The stable isotope ratio (d13C ) values are expressed with ref-erence to an international standard in parts per thousand (permil or %).Biogenic methane, on average, contains a greater proportion of isotopicallylighter carbon (i.e. is more depleted in 13C) than thermogenic methane.Further differentiation is possible, but one potential problem with relying onstable carbon isotopes is that if the gas migrates from its source it mayundergo other changes, such as oxidation, that can result in changes in theratios and introduce the risk of misinterpretation.24 The d13C of thermo-genic methane lies in the range 110 to 55% and biogenic gas in the range55 to 20%.25In addition to methane and other light hydrocarbons, shale gas may also

contain small amounts of carbon dioxide, oxygen, nitrogen, hydrogen sul-fide, rare or noble gases (argon, helium, neon and xenon) and radon. Theconcentrations of these gases are generally low and do not present a majorhazard, except for radon which is radioactive. This will require appropriaterisk assessment and management.

5 Contaminant Pathways and Receptors

As an activity that takes place on and below the land surface and involvesthe use of potential pollutants, there is the potential to pollute groundwater.


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This pollution may occur as a result of poor practice, accident or unexpectedcircumstances that lead to pathways being created between the source ofthe pollution and the receptor. Receptors in this case include humans,groundwater, surface water and ecosystems. The pathways may becreated by:

Run-off or infiltration to the ground arising from spills and leaks of li-quids, chemicals and operational wastewater being transported to/fromthe site, stored on the site or during use in the hydraulic fracturingprocess/operation;

Uncontrolled releases of drilling muds/fluids into non-target geologicalformations containing groundwater and aquifers during installation ofexploration and production wells;

Migration of drilling muds/fluids or hydraulic fracturing fluids injectedat high pressures along natural geological discontinuities (faults andfractures);

Hydraulic fracturing creating interconnected fractures that extend be-yond the intended zone and provide migration pathways for drillingmuds/fluids, hydraulic fracturing fluids and formation water to leakinto non-target geological formations containing groundwater andaquifers;

Well failure arising from poor construction or loss of well integrityduring the lifetime of operation, including damage resulting from in-duced seismicity or other ground movement, leading to uncontrolledreleases of drilling muds/fluids, hydraulic fracturing fluids and for-mation waters into non-target geological formations containinggroundwater and aquifers; and

Existing infrastructure such as wells (active or abandoned), mineworkings and adits providing pathways for drilling muds/fluids,hydraulic fracturing fluids and formation water to leak into non-targetgeological formations containing groundwater and aquifers.

Multiple pathways may potentially exist and interact. It is thereforeessential that an appropriate level of risk assessment is carried out andeffective risk management procedures established. This will require acombination of actions, some of which are engineering-based and will beaddressed by ensuring that industry good practice is adopted; others willrequire site-specific investigation, characterisation and monitoring.

5.1 Natural Sub-surface Pathways

Myers26 identifies two possible mechanisms by which pollutants may mi-grate from fractured shale to shallow aquifers along natural pathways in thesub-surface: advective transport through the rock matrix, and preferentialflow through fractures and other discontinuities. He suggests that there issubstantial geological evidence that natural hydraulic gradients can drive


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contaminants to near the surface from deep evaporite sources. The model-ling presented shows it would take tens of thousands of years for pollutantsto migrate from depths of 41.5 km to the near surface if transport wasthrough the rock matrix alone. However, hydraulic fracturing of the rock, theintroduction of fractures and substantially higher pressure gradients couldreduce transport times to tens or hundreds of years under certain con-ditions. The estimates presented by Myers have been challenged by Saiersand Barth27 who say the approach used is far too simplistic and neglectscritical hydrological factors, such as fluid density gradients, fluid saturationand temperature. They do, however, recognise that there is a need toundertake more modelling to derive estimates of fluid migration and pol-lutant transport along natural (and induced) pathways from shale gas-bearing rocks.In the UK there is considerable uncertainty about the geophysical,

chemical and hydrogeological properties of the rocks which comprise thenatural pathways in the areas that are being considered for shale gas.Knowledge and measurement of sub-surface properties below depthsof about 100 m is extremely sparse in all but a very few locations, and sothere is an important need to fill this knowledge gap to allow developmentof sensible conceptual and mathematical models of fluid movementand behaviour in the deep sub-surface before shale gas exploitationproceeds.

5.2 Induced Fractures

A frequently expressed concern about shale gas development is thathydraulic fracturing might create fractures that extend well beyond the tar-get formation to overlying shallow groundwater and/or aquifer formations.These would then allow migration of pollutants such as methane, highlysaline and mineralised formation water, and fracturing fluids from thetarget formation to contaminate drinking water supplies.28

If hydraulic fracturing is carried out at the depths that are being sug-gested, e.g. greater than 1 km, these fractures would have to propagateconsiderable distances and, more often than not, through geological se-quences that comprise rock types with different physical properties. Becauseof this, propagation of fractures over these distances is highly unlikely as aresult of shale gas hydraulic fracturing operations. A report for NewYorkState concludes that fracking is unlikely to create a pathway beyond thefractured zone and the post-fracking reversal of pressure means that fluidswill migrate back to the well.29

Work has been carried out to compile information on both natural andshale gas fracture propagation.30,31 Data were examined from each of themain shale gas formations in the USA and a statistical analysis carried out.The maximum recorded upward propagation of fractures was approximately588 m and the calculated probability of a fracture extending more than350 m was around 1%.


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5.3 Drilling and Well Integrity

The risks associated with well design and drilling have to be carefully con-sidered and take into account the purposes of the well, the geological for-mations that will be drilled and the materials being used/produced duringand after drilling. Any structure that penetrates freshwater aquifers, such asa well, has the potential to introduce a preferential pathway that could leadto contamination if pollutants are allowed to leak.14 Control measures mustbe put in place to mitigate any identified risks. These include installation ofmultiple casing and cement bonded wells, blow-out preventers and en-vironmental monitoring.The key hazards associated with the drilling operation that could po-

tentially lead to groundwater/surface water contamination are: loss ofdrilling fluids to the surrounding geological formation(s) (leak-off);well blow-out as a result of gas or fluids under high pressure being en-countered in the well bore; and spillages of wastes and chemicals on thesurface.During drilling of the shallow geological formations, drilling fluids

which aim to minimise the risk of groundwater contamination are gener-ally used. Examples include the use air and/or water. The well casing andcement that holds it in place provide the seal(s) that is (are) of vital im-portance both during the drilling phase and then for maintaining the in-tegrity of the well during its lifetime. Failure of the cement or casingsurrounding the wellbore poses a significant risk to groundwater. If theannulus is improperly sealed or the seal fails, natural gas, fracturing fluidsand formation water(s) containing high concentrations of pollutants maybe communicated directly along the outside of the central wellbore be-tween the target formation, drinking water aquifers and layers of rock/groundwater in between.Studies that have looked at well integrity failure reveal considerable vari-

ation in failure rate. Work by Schlumberger estimated that by the time a wellis 15-years old there could be a 50% chance of failure.32 CIWEM reportedthat 6 to 7% of new wells in Pennsylvania have compromised structuralintegrity,33 and a more recent review showed that rates of wells with integrityissues ranged from 2.9 to 75%, with the lower rates reflecting wells drilledsince 2010 in Pennsylvania.46

Another potential cause of well integrity failure is as a result of groundmovement, which includes damage induced by seismic activity triggered byhydraulic fracturing. Casing deformation is relatively common in deephydrocarbon wells due to geological processes and differences in theproperties of adjacent geological formations. This may result in horizontalshearing and subsequent deformation (buckling) of the well casing. AtPreese Hall in Lancashire the seismic events which were triggered by thehydraulic fracturing of the exploratory well are also believed to have led todeformation of the well casing as a result of seismically induced rockshear.34


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5.4 Surface Accidental Releases of Liquids and Chemicals

Shale gas well drilling and hydraulic fracturing requires a period of 12months of intense activity around the well, during which spillages or leakageof polluting substances may occur. Activities which have been identified ashazardous include: re-fuelling of diesel tanks, bulk-chemical or fluid trans-port and storage, equipment cleaning, vehicle maintenance, leaking pipework, drilling mud/cement mixing areas, wastewater storage and transport.As significant volumes of fluid and chemicals are stored/mixed/used on sitethere is potential for either direct run-off to drains, ditches and other watercourses, or infiltration to ground which may adversely impact surface waterquality and ecology or may lead to localised groundwater pollution.In the United States, storage or retention pits are frequently used for

holding freshwater and/or wastewater. These are unlikely to be allowed inthe UK and fluids will be required to be held in storage tanks. Tanks can alsobe used in a closed-loop drilling system. Closed-loop drilling allows for there-use of drilling fluids and the use of lesser amounts of drilling fluids.Closed-loop drilling systems have also been used with water-based fluids inenvironmentally sensitive environments in combination with air-rotarydrilling techniques. The containment of fluids reduces the risk of leakageand is likely to represent standard practice if shale gas exploitation goesahead in the UK.

6 Risk Assessment, Regulation and Groundwater Protection

There is a well-established oil and gas industry in the UK and more than2100 wells have been drilled since 1902 for hydrocarbon exploration or ex-ploitation.35 It is a regulated industry with several regulatory bodies andagencies responsible for the different aspects of the operation (see Table 7).Whilst shale gas is a new development in the UK, many of the regulationsand procedures will be applicable or directly transferable. The key bodiesthat will be involved are the Department of Energy and Climate Change(DECC), the relevant environment agency (Environment Agency, ScottishEnvironment Protection Agency, Natural Resources Wales or NorthernIreland Environment Agency), Healthy and Safety Executive and local (andmineral) planning authorities. To oversee the safe and responsible devel-opment of unconventional oil and gas and ensure co-ordination of activitiesacross the UK, the Government established the Office of Unconventional Gasand Oil (OUGO) in 2013. One of the first outputs from OUGO is a roadmapthat provides an introduction to and guidance on the planning and per-mitting process for unconventional oil and gas exploratory well drilling.36

OUGO recognises that, as the industry is in its infancy in the UK, theroadmap will need to be revised as legislation develops, new regulations areintroduced, or when best practice becomes established.Currently the roadmap does not address the full range of environmental

risks and risk management requirements. For example, it does not cover


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Table 7 List of government bodies and agencies with responsibilities for regulating aspects of shale gas exploitation, water supply andenvironment protection in the UK.

Government department/devolved department/agency

England Wales Scotland Northern Ireland

Hydrocarbons exploration &development licensing, emissionstargets, environmental riskassessment

Department of Energy and Climate Change (DECC)through PEDL (Petroleum Exploration andDevelopment Licenses) rounds

Energy Division,Department of Enterprise,Trade & Investment (DETI)

Planning permission & accessnegotiation, environmentalimpact assessment

Minerals Planning Authority (MPA) Planning & LocalGovernment Group, Departmentof the Environment (DOE,Northern Ireland)

Department for Communities and LocalGovernment (DCLG) Local Planning Authorities

Permission to penetrate coal seams Coal Authority

Notification of drillingoperations, well design andconstruction inspection

Health and Safety Executive (HSE) Health and SafetyExecutive NI (HSENI)

142Robert

S.Ward,

Marianne

E.Stuart

andJohn

P.Bloom

field

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Induced seismicity risk British Geological Survey (BGS)Intention to drill, abstractionlicensing, environmentalpermitting & Water Framework/Groundwater Directiveobjectives, fugitive emissions,environmental monitoring

EnvironmentAgency

NaturalResourcesWales

ScottishEnvironmentProtectionAgency

Northern IrelandEnvironment Agency

Hazardous substances Joint Agencies Groundwater Directive Advisory Group (JAGDAG)Expert advice on health impactsto environment agencies

Public HealthEngland

Public HealthWales

Scottish PublicHealth Network

Public HealthAgency Northern Ireland

Drinking water quality regulation Drinking Water Inspectorate Drinking WaterQuality Regulator Scotland

Drinking Water InspectorateNorthern Ireland

Water industry capital investmentand water pricing

Water Services RegulatoryAuthority (OFWAT)

Water IndustryCommission for Scotland

Utility Regulator

Environmental effects Department for Environment,Farming & Rural Affairs (Defra)

Scottish ExecutiveEnvironment & RuralAffairs Department (SEERAD)

Department ofthe Environment(DOE, Northern Ireland)

143The

Hydrogeological

Aspectsof

ShaleGas

Extractionin

theUK

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groundwater monitoring requirements both in terms of establishing a base-line and during drilling/operation of the well(s). This contrasts with the at-tention given to induced seismicity and associated monitoring requirements.The Environment Agency has gone some way to identify the environmental

risks and has completed a high-level risk assessment.37 It is also in theprocess of developing technical guidance to explain which environmentalregulations apply to operations to explore for on-shore oil and gas in Eng-land and the permissions that need to be obtained. This will need to bedeveloped further into more site-specific guidance for shale gas exploration/exploitation risk assessments to ensure adequate controls are implementedand risks managed effectively. The different sourcepathwayreceptor com-binations that will need to be considered are illustrated in Figure 6.The environmental regulators each have polices for the protection of

groundwater.3840 These set out the legal (EU and UK) requirements tomanage and protect groundwater (and the associated wider environment);the regulatory framework; environmental objectives; management andrisk assessment tools/methodologies; and the supporting monitoringrequirements.The UK groundwater protection strategies adopt a risk-based approach in

general but apply a precautionary approach in some instances where theconsequences of impact may be so severe that the uncertainties associatedwith assessing risk are considered to be too great. An example of this is theinner source protection zone (or SPZ1) for public water supply abstractions.The precautionary principle is applied and certain activities (e.g. landfilling)

Figure 6 Conceptualised illustration of pollutants, pathways and receptors associ-ated with shale gas operations.


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are prohibited within SPZ1. The Environment Agency has indicated that itwill also object to any shale gas infrastructure being located in SPZ1.41

In order for an operator to carry out an activity that may potentially impactgroundwater (surface water or habitats) they must demonstrate to the en-vironment agencies that they have assessed the risks to groundwater as partof the environmental permitting process. Whilst the environment agenciesalready have directly applicable procedures (as they permit many other in-dustrial activities) there are unique aspects of shale gas exploitation thatwill need to be considered. This is particularly the case because the risksto groundwater are not only from activities on (or close to) the land surface top down but also from below bottom up. This requires a new approachto groundwater vulnerability assessment and risk assessment. The BritishGeological Survey in partnership with the Environment Agency is currentlymapping the 3D spatial relationship between shale gas source rocks andthe principal aquifers of the UK as a preliminary step in this process(www.bgs.ac.uk/research/groundwater/shaleGas/aquifersAndShales).

7 Evidence of Shale-gas-related Groundwater Contamination

There have been only a few published peer-reviewed scientific studies thathave assessed the impact of shale gas extraction on groundwater, but thenumber is increasing as concern has grown about the environmental impactof the industry. As the United States has seen the most rapid development ofshale gas it is inevitable that the focus of most studies is here. One of thechallenges that has emerged is the fact very little, if any, baseline monitoringtook place before development, which has led to considerable uncertainty inattributing contamination directly to shale gas as the specific cause(s). In theUK, as development has not yet taken place, there is an opportunity to es-tablish a pre-industry baseline and the British Geological Survey (BGS) hasinitiated such a study.6 It is expected that the industry will also be requiredto undertake more localised baseline monitoring (as well as on-goingmonitoring) around any exploration/production sites as a condition of theirenvironmental permit.From the studies in the United States that have been published, the most

common problems appear to be related to well integrity, where poor installationof wells and/or their degradation over time has been identified as a potentialmechanism for contamination of shallow aquifers. As described earlier, studiesof available datasets on well integrity show the extent of the problem.46

In 2007, a well that had been drilled almost 1200 m into a tight sandformation in Bainbridge, Ohio, was not properly completed and methanemigrated upwards to contaminate a shallow aquifer and private water sup-ply. A build-up of methane in the basement led to an explosion which alertedstate officials to the problem.42

In aquifers overlying the Marcellus and Utica shale formations of north-eastern Pennsylvania and upstate New York, Osborn et al.43 have docu-mented evidence for methane contamination of drinking water and


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attributed this to shale gas extraction. In active gas-extraction areas, averageand maximum methane concentrations in drinking-water wells (19.2 and64 mg CH4 per litre, respectively) increased with proximity to the nearest gaswell. At these concentrations there is a potential explosion risk if gas isallowed to accumulate in a confined space. In contrast, samples fromgroundwater in non-extraction areas (no gas wells within 1 km) withinsimilar geologic formations and hydrogeological settings concentrationsaveraged only 1.1 mg l1. These results and conclusions were disputed by anumber of other scientists on the basis that no baseline was available and itwas known that groundwater in the area contained naturally elevated con-centrations of methane. The study was extended by Jackson et al.,5 with alarger number of samples and also with additional isotopic and geochemicalanalysis. This further work concluded that a sub-set of drinking water wellswere contaminated as a result of drilling operations and that this was likelyto be due to poor well completion (integrity).Another well-publicised study at Pavillion, Wyoming,44,45 investigated

contamination of shallow groundwater supplies in an area with a con-siderable number of oil and gas wells (169) that had been hydraulicallyfractured. The study, which was led by the US Environment ProtectionAgency (USEPA), collected samples from both deep monitoring wells andshallower domestic wells. The study found that concentrations of dissolvedmethane in domestic wells generally increased the closer they were to gasproduction wells. An analysis of data on hydrocarbon well completion re-vealed a catalogue of problems with production wells having poor cementbonding or even no bonding present at all or over considerable lengths ofthe wells. Whilst these would almost certainly contribute to migration ofcontaminants to the shallow aquifers, other causes for concern and alter-native pathways were identified. It was found that hydraulic fracturing andhydrocarbon exploitation took place at relatively shallow depths, from 372 mbelow ground level, whilst some drinking water abstraction wells in theoverlying aquifer were as deep as 244 m. This means that there is onlylimited vertical separation between the two. A further problem identified wasleakage and infiltration to shallow aquifers from storage pits on the surfaceused for storage/disposal of drilling wastes, produced-water and flowbackfluids. The study is continuing but the USEPA have now, somewhat con-troversially, handed over responsibility to the State of Wyoming.It is highly unlikely that such a situation would arise in the UK because of

the strict controls that would be applied. However, the experiences from theUnited States serve as a clear indication of why, and what can happen if,things go wrong.

8 Conclusions

The UKmay possess considerable reserves of shale gas with shale underlyinga significant proportion of the UK, but as yet there has been very littleexploratory drilling to confirm the resource potential.


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In some parts of the UK the areas likely to be exploited for shale gas areoverlain by significant aquifers used for drinking water supply and forsupporting baseflow to rivers. The vulnerability of groundwater (and thewider water environment) to shale gas operations must therefore be takenvery seriously.Groundwater may potentially be contaminated by extraction of shale gas,

both from the constituents of shale gas itself, from the hydraulic fracturingfluids, from flowback/produced water which may have a high content ofsaline formation water, or from drilling operations.A rigorous assessment of the risks is required and appropriate risk man-

agement strategies need to be developed and implemented if the industry isto become established. It is likely that, due to environmental sensitivities,there will be some locations that shale gas exploitation will be consideredunacceptable. To what extent this may affect the economic viability of theindustry is unknown. In fact, there are very many unknowns as we are at thepreliminary stages of exploration.Because we are still at a very early stage we can take advantage of this and

ensure that progress is made in a controlled way. We must identify andunderstand the risks to groundwater from shale gas and establish a fullyinformed risk management strategy for the industry. Experience from theUnited States has illustrated what can go wrong if this is ignored. We mustnot look back in 2030 years and regret not taking the actions we have theopportunity to take now.

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the hydrogeological aspects of shale gas extraction in the uk

Documents

gasrich shale

constituents of shale

extraction of shale

viable source of gas

bloomfieldabstractthe

north sea gas reserves

ifthe industry

shalegas exploitation